Isotactic polypropylene is capable of crystallizing in several crystal forms. The alpha or monoclinic form is the most stable and prevalent one. The beta or hexagonal form is occasionally found in commercially available polypropylene, usually at low levels.
The relative proportion ("K") of beta form crystalline in the polypropylene specimen can be determined by X-ray diffraction and expressed by the empirical expression: ##EQU1##
Wherein H.sub.110, H.sub.040 and H.sub.130 are the heights of the three strong peaks (100), (040) (130) of the alpha-form respectively and H.sub.B is the height of the strong beta-peak (300).
The K value is zero in the absence of the beta form and one if only the beta-form is present in the polypropylene.
In typical commercial products produced through injection molding, spinning filament and extruder shaping have spherulitic morphology, in which there is only a very small percentage of beta-crystallinity. Until recently, there has been no reliable way to produce enough beta-form for detailed studies. H. J. Leugering at Hoechst has reported that small amounts of a quinacridone colorant, permanent REd E3B, preferentially nucleated the beta-form. This compound is one of a few effective well known beta-uncleating agent. But even when polypropylene is crystallized from the melt in the presence of E3B, it is still difficult to obtain a material with a relatively high amount of beta-form. Ullmann and Wendorf have reported that the K value of polypropylene samples, crystallized in the presence of E3B, is less than 0.6. Moos and Hilger have succeeded in producing a sample with a K value as high as 0.85 by melt-extrusion of a special powder grade of polypropylene with E3B in a twin-screw extruder, but no further studies have been conducted. A method was disclosed in JP Kokai No. 57-172943 for producing the beta-form of isotactic polypropylene, which comprises mixing a quinacridone colorant with a polymerization catalyst, and then polymerizing the propylene in the presence of the foregoing mentioned mixed catalyst. The thus produced polypropylene was melt-extruded and then crystallized at a cooling rate of 10.degree. C./min. The sample was reported to contain 63-88% of beta-form, as measured by a differential scanning calorimeter (D.S.C.). The D.S.C. method, however, is not a precise method for measuring beta-form content in our opinion, based on our experience the content of beta-form measured by D.S.C. may be overestimated. Furthermore the beta-phase polypropylene thus obtained is red in color, thermally unstable and easily transform into the alpha-form during heating.
Another beta-nucleating agent described in the literature is the bi-sodium salt of O-phthalic acid and to a lesser degree isophthalic and terephthalic acids. These beta-nucleating agents are less effective than the E3B and no quantitative data for the K value is given.
The inventions of the present invention have unexpectedly found a more effective beta-nucleating agent for polypropylene which is able to produce a much higher content of beta-form polypropylene material under typical processing conditions.